The present invention relates to a method and apparatus for measuring the transmission of liquid vapors through membranes. The permeability of film membranes to moisture and other vapors is considered to be a very important factor in the ability of the membrane to act as a barrier, for sealing flavors in packaged materials and for preventing degradation of the product wrapped inside the film material. Among the types of film membrane which are used for packaging materials are plastics, coated papers and various combinations of multi-layered packaging.
One accurate technique for measuring the ability of a film material to serve as a barrier to moisture and various liquid vapors is the gravimetric technique, which involves the steps of actually weighing a closed container having an initial moisture content, where the container has a wall made from the test film material, the loss of weight over time being attributable to the loss of moisture through the wall of film material. A test device known as a Thwing-Albert test cup has long been used for this purpose. This device consists of a cup having a removable cover ring, where a film sample is clamped over the open top of the cup by the ring, and the entire assembly is periodically weighed on an accurate scale. As moisture escapes through the film material the cup loses weight, and after a period of time the total weight loss can be translated into a numeric value which represents the transmission rate of moisture through the film material. The Thwing-Albert test cup is a relatively simple device, consisting of a cover ring having a number, usually six or eight, of threaded fasteners located at equiangular positions about the ring, wherein the fasteners are threaded into the lower cup body and are used to clamp the edge of the film material tightly against the upper rim of the cup. This device is usually accurate for measuring permeation rates through film material, having an accuracy in the range of 20-100 milligrams per day with an initial permeant weight of 30-40 grams inside the cup. The device is not sufficiently accurate for measuring permeation rates at very much lower levels, for the readings are compromised by extraneous leakage due to uneven clamping of the film material, including leakage around the gasket materials used to seal the film across the top of the gravimetric cup.
As technology has progressed, the barrier capabilities of film materials have greatly improved over the years, and materials are now being developed which have permeation rates far below the range of accuracy which the Thwing-Albert test cup is capable of measuring. Weight loss accuracy of around 1-5 milligrams per day is now desirable, and test devices capable of operating with this accuracy are very much needed.
It is an object of the present invention to provide a test apparatus which enables the precise and repeatable measurement of permeation of liquid vapors through a film material below a rate of about 5 grams per day, with an accuracy of better than about one milligram per day.